1. Field of the Invention
This invention relates to a process for forming a lead film which is chemically stable, stronger in adhesion strength with respect to a substrate, and high in packing density.
2. Description of the Prior Art
Lead is an important material in a field of electrochemistry and is advantageously used in electrodes in industrial electrolyses and lead-batteries. It is also used for radiation shielding materials, solder materials and overcoat alloy for cables, because of its larger atomic number and high density. Furthermore, it is an important material adapted for use in superconducting elements, such as, for example, Josephson element, because it exhibits superconductivity under an ultra low temperature of less than 7.degree. K.
When applying lead as a material for fabricating the superconducting element, it is necessary to form a thin lead film. In the prior arts, such a thin lead film is only produced by vacuum evaporation. However, the thin lead film formed by the vacuum evaporation has smaller density as compared with a solid-state lead, and is easily oxidized and bonded to O.sub.2 if it is exposed in air after the formation of the film, and then it is converted into chemically unstable PbO or PbO.sub.2. Furthermore, the thin lead film produced by the vacuum evaporation has such a mechanical defect that adhesion strength with respect to a substrate is not strong enough, which causes the lead film to easily peel off from the substrate. Moreover, it is difficult to control the film thickness and crystal orientation axis when forming the thin lead film. In view of these disadvantages and others, the thin lead film produced by the conventional vacuum evaporation is not adapted for use in the superconducting element. In the superconducting element, it is important not to cause the lead film to peel off due to a temperature cycle between ultra low and room temperatures which the superconducting element undergoes.
In addition, the lead film has tremendous utility in the field of electrochemistry. For instance, if electrodes of various batteries could be made of the thin lead film, it is possible to make various apparatuses using the battery smaller in size and light in weight and to save the materials for producing these apparatuses. However, the production of such an apparatus has not yet been successful, because of the defects inherent in the thin lead film formed by the vacuum evaporation.